If you've been hearing a lot about echo cancellers these days, you
may also be wondering what they are, and why we need them. Don't underestimate the need for these obscure technical devices, as
they play a major role in our perception of network quality. Note that much of what is
said in this article is an oversimplification of the complex echo cancellation
process. If you want to learn more, I suggest you read up on them either on the Internet
or at your local library/bookstore.

What is an Echo Canceller and
Why do we Need It?

Most (though not all) telephone calls in
North America, whether they originate from a landline or a cell phone, eventually
terminate on a two-wire system, or pass through a two-wire system at some point.
This means that we have just two wires carrying our conversation in both directions. To make this
work the audio from both ends of the
line must travel together on those two wires. The upshot of
this is that phones at both ends hear both sets of audio through their
earpieces. Before the days of digital wireless phones this rarely presented a problem. In
fact, hearing your own voice in the earpiece was reassuring, as it let you know the phone
was working. There is even a term to describe this. We call the sample of your own voice a
"side tone".

But if a side tone is such a good idea in a landline phone, why
isn't it an equally good idea in a digital wireless phone? Before I can answer that I must
first bring you up to speed on how a digital wireless phone
transmits audio. The first step of the process is to translate your voice into a stream of
0s and 1s. This process is known as digitization. This process is not unique to wireless
communications however, as just about every single landline phone system converts voice
into digital anyway.

The key difference comes in the next step. RF bandwidth is a
precious resource, and wireless carriers must do everything they can to stuff as many
callers into as small a chunk of bandwidth as possible. The stream of bits that result
from digitization is horrendously wasteful. The landline guys don't care about this, since
they don't have a bandwidth restriction to worry about. We can radically reduce the stream
by applying a compression algorithm to the bits. Unlike computer files, which must
compress and expand without a single error, voice compression
has a lot more leeway. We can selectively "throw away" parts of the audio to
achieve levels of compression that far exceed that which you can get with PKZIP.

Like all compression algorithms however, the real data must be allowed to
"run ahead". This gives the algorithm all the data it needs to perform the
complex task of compression. Typically the audio is allowed to run ahead by
approximate 1/8th of a second. You can hear this delay by using your wireless phone to
call your landline. Talk into the wireless phone and listen to yourself on the landline
phone.

Here's where the problem comes in. Your voice is delayed by 1/8th of
a second before it is transmitted by your phone to the network. The network then delivers
this audio stream to the landline network where the two-wire system echoes it back to the
network. Finally, the network retransmits the audio back to you, but adds a further 1/8th
of a second delay in the process. Your brain can tolerate a small amount of delay between
the words leaving your mouth, and what it hears at your ears. However one quarter
of a second
is simply way too long, and the results of trying to speak under those conditions are
comical at best.

The Ontario Science Center in Toronto had a demonstration
of this for quite some time. In this demo the visitor put on a pair of headphones and then tried to
read "Mary had a little lamb" into a microphone. A tape loop provided the necessary
delay so that your words arrived at the headphones about a half second after you
spoke them. Needless to say this was hilarious for your friends to witness. Can you honestly
say you want all your callers laughing at you?

An echo canceller is a piece of technology that does away with this
problem. Its function is easy to describe, but horrendously difficult to achieve. The echo
canceller literally removes your voice from the returning audio stream without
removing the audio coming from your caller. This is difficult because the canceller must
produce a perfect inverse rendition of your audio so that only
your voice is removed. Although this technology has been in development for decades, it
has only recently become good enough to be called "excellent".

Echo canceller technology must be tuned to work correctly
under the expected conditions. Various models are capable of on-the-fly tuning
that allow them to adjust to the conditions in real time. Others seem to lack
this ability completely, or simply don't implement it well. Each wireless
provider uses echo cancellers that are either built into their switches, or are
purchased independently. The quality of the echo canceller will determine the
quality of your call.

When echo cancellers don't work right they produce a
variety of unwanted side effects. The most obvious is that you hear your own
voice echoed with a 1/4 second of delay. Depending upon the volume of the echoed
sound, this can range from mildly annoying to downright disruptive. A more
common symptom however is that the canceller does its job, but it does it too
well (so to speak). In other words, it cancels your voice AND it cancels part of the voice of
your caller. In this situation you'll find that talking over your caller will
obliterate them, or that taking a call in a noisy environment can sometimes
cause continuous damage to the audio that simply makes it sound
distorted.

If you hear symptoms such as the those I've described
above, you are probably hearing the side effects of a poorly tuned or poorly
designed echo canceller. There isn't much you can do about this, short of
complaining to your provider, or switching to a different provider. Phones whose
microphones are closer to your mouth will likely suffer less problems in noisy
environments than those having microphones far away from your mouth (due to the
latter requiring great microphone sensitivity). Phones with louder outgoing
audio will suffer more greatly in the presence of a malfunctioning echo
canceller.